We are applying stable water isotope labeling followed by MRI for in vivo visualization of tumors in a pre-clinical model. We hope to implement such imaging in the clinical setting. Our imaging approach is non-radioactive and the labeling with stable water isotopes has already been shown to be safe at the doses required for imaging. Furthermore, our methodology may be useful for visualizing immunotherapeutic products if they undergo ex vivo labeling during manufacture, for example during the cell culture processing.